The objective of this research project is to solve the crystal structure of Cd, Zn metallothionein. The means of study will be X-ray crystallography. The protein to be studied is Cd, Zn metallothionein isoform II (Cd, Zn MTII) from rat liver. Excellent quality single crystals are available in good yield. The crystals are tetragonal, space group P41212 (P43212), with a = b = 31.0, c = 120.0 Angstrom, and one molecule in the asymmetric unit. The crystals diffract uniformly to at least 2.2 Angstrom resolution. The protein is a single polypeptide of 61 residues, MW 6500, containing 20 cysteines and 7 metals arranged in two polynuclear clusters of average compositions Cd4 and Cd1Zn2. Samples of rat liver Cd, Zn MTII for crystallization will be prepared using a standard procedure. The complete amino acid sequence of this isoform will be determined. Samples of MTII with metal compositions differing from the native will be prepared by reconstitution methods and used for crystallization experiments. X-ray diffraction data will be collected: with a single crystal diffractometer equipped with Cu and Cr target X-ray tubes; and/or by oscillation photography using cameras mounted on either of two Cu-target rotating anode instruments; and/or at the Brookhaven National Synchrotron Light Source using a diffractometer or oscillation camera. The structure will be solved using the anomalous scattering effects from Cd, Zn and/or S in the native crystals at the multiple wavelengths. Data will also be collected with crystals of reconstituted protein containing other metals, and with crystals soaked in heavy atom compounds.